1. Field of the Invention
The present invention relates to a point-to-multipoint data communication network, and more particularly, to a method for transmitting data and compensating for a propagation delay in a point-to-multipoint data communication network.
2. Description of the Related Art
Point-to-multipoint data communication networks sharing transmission media include a Hybrid Fiber & Coaxial (HFC) network using a cable modem, and a Broadband Wireless Local Loop (B-WLL) network that is a wireless access network. Downstream refers to a data transmission direction from a network access point to stations while upstream refers to a data transmission direction from stations to the network access point. In a point-to-multipoint data communication network, a downstream channel transmits data through broadcast and an upstream channel transmits data by sharing media under the control of the network access point. Especially, the upstream channel transmits data at a predetermined time using a specific protocol for an effective access to media. For the timely transmission of data, stations need to be correctly synchronized with the network access point in a network.
The point-to-multipoint data communication network includes the network access point and multiple stations. The network access point broadcasts a time information message to stations periodically so that they can be synchronized with the network access point. After receiving the broadcasted time information message, the stations synchronize the time of their own timers with that of the network access point. In addition, since the propagation delay between the network access point and stations are compensated for in the ranging process, the network access point can transmit and receive data accurately at a desired time. To maximize the utilization efficiency of shared media, the network access point calculates transmission timing of each station by means of scheduling based on various parameters related to each station, prepares transmission/receiving timing information for media access and broadcasts the prepared transmission/receiving timing information to each station by transmission frame. Upon receiving the transmission/receiving timing information, each station sends data to the network access point at the time of summing an offset time and an allocation start time included in the transmission timing information.
According to the conventional methods for transmitting data and compensating for a propagation delay, to maintain and adjust the network synchronization for determination of an accurate transmission timing, the network access point should broadcast a synchronization (SYNC) message periodically to stations. In addition, a ranging message should be exchanged periodically in order to compensate a delay due to the propagation distance between the network access point and stations, resulting in a high overhead. Therefore, since media were not totally used to transmit data, the entire transmission capacity was impacted.